1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device including a cathode that has high light transmittance.
2. Description of the Related Art
Organic light emitting devices, which are self-emitting devices, have high brightness compared to liquid crystal display devices and can also be thinner than liquid crystal display devices because they do not use backlight units.
An organic light emitting device has a structure that includes an anode, an organic layer, and a cathode, which are sequentially stacked on a substrate on which pixel circuits are arranged and can be embodied as, for example, thin film transistors. Organic light emitting devices can be classified into two emission types according to the direction of light emission. These are a top emission type and a bottom emission type. In a top emission type organic light emitting device, light is emitted to form an image in a direction away from a substrate on which the organic light emitting device is disposed, that is, towards a cathode. Thus, the top emission type organic light emitting device can have a high aperture ratio compared to a bottom emission type organic light emitting device in which light is emitted to form an image in a direction towards the substrate, thereby having high light transmittance. Although the cathode, which is formed on the top of the organic light emitting device, needs to be transparent, the improvement of transparency of the cathode is limited. That is, while the cathode needs to have lower work function than the anode, a material having a low work function, such as a metal, generally has low light transmittance.
A conventional transparent cathode is formed of a thin film of a metal having a low work function. However, since the metal has a low light transmittance, improvement of light transmittance is limited.
As a result of the limitations on improving light transmittance of the top emission type light emitting structure, a micro-cavity which amplifies light emitted from a light emitting layer has been reported as a possible solution. However, since different colors have different ideal thicknesses of the micro-cavity, the thickness of the organic layers interposed between the anode and the cathode will vary. That is, since light is emitted by exitons formed by recombination of holes injected from the anode and electrons injected from the cathode in the organic light emitting layer, the thicknesses of the organic layers, particularly of a hole or electron injection layer, or a hole or electron transport layer, have been varied to control distances from exitons and resonance for each color. An independent mask is required for each color to vary the thicknesses of the deposited organic layers. However, such independent deposition has the disadvantages of further complicating processes and increasing costs for the manufacturing process.
Further, as the resolution of display devices increases, forming a more finely pitched mask pattern becomes increasingly difficult and independent deposition becomes increasingly difficult to apply to large area display devices.
Further, the thickness of the organic layers cannot be determined simply by optical efficiency since electrical characteristics of the device may worsen when the thickness of the hole layer or the electron layer varies.